In metal making processes, such as steel making, a layer of slag comprising metal impurities lies atop the surface of the molten metal held within a receptacle. When the molten metal is drained from the receptacle, separation of the slag and molten metal improves the quality of steel being discharged, as the flow is not contaminated by the slag. One example for maintaining the separation includes providing a receptacle with discharge opening located in the bottom wall of the vessel so that the opening is in fluid contact only with the layer of molten metal in the production vessel and is separated from the slag at the top surface of the molten metal. However, in previously known vessels, the flow of molten metal through the discharge opening, such as a tap hole or a pouring bore which may in turn, be provided with a sleeve or a nozzle with an opening, causes a vortex which introduces a swirl to the molten metal within the vessel above the discharge opening which can mix the slag with the metal.
As the level of the contents of the vessel decreases, a minor swirl is imparted to the fluid, but may not affect the separation between the slag layer and the steel layer. However, when the fluid level reaches a certain depth which is dependent upon the size of the discharge opening, the vortex forms a funnel which sucks the slag layer down through the center of the vortex and into the discharge opening along with the high quality molten metal. At this point, the quality of the strand being formed is affected by contamination of the pour with the slag. As a result, flow may be halted before substantial contamination reduces quality of the product. However, the remaining metal is removed from the vessel as scrap or hardens in the vessel for subsequent remelting. Unfortunately, the level of fluid at which the vortex suction effect occurs is relatively high, whereby a substantial amount of high quality molten metal remains trapped within the vessel and must be abandoned from the process.
One previously known improvement provides a refractory body adapted to inhibit the vortex effect and permit a greater quantity of high quality molten metal to be poured from the vessel without the intermixture of slag. The body generally comprises a tapered, polygonal body having means for supporting the body at the interface of the layer of slag and molten metal to inhibit the suction effect which occurs when the vortex is formed at the critical level. Preferably, the body is made with a refractory material with a specific gravity that makes it buoyant but provides a submerged portion as it is self-supported at the slag layer/metal interface. Such a body extracts sufficient energy from the vortex to avoid the formation of a suction-effect funnel and prevents intermixture of the slag and the molten metal. In addition, when the apex of a buoyant tapered body is oriented directly downward toward the discharge opening so that as the apex approaches and begins to enter the nozzle opening, a throttling effect is initiated. The change in flow volume by throttling to provides a means for detecting that the level of slag is approaching the opening. However, the throttling effect of reduced flow may not occur as desired where the shape of the body has been changed substantially due to the harsh temperature, chemical and kinetic conditions occurring in the vessel. Moreover, the throttling effect is often detected by observation, and may be difficult to discern under the harsh environmental conditions of the processing equipment.
As a result, substantially less high quality metal may be poured than is available to assure that the quality of the discharged metal remains high. Substantial energy input is required to reheat metal which has hardened within a vessel or dumped out with slag removal process. In addition, it has heretofore been difficult to detect or sense when the level of molten metal is at the critical level in the vessel. Since the suction action of the vortex draws the slag into the center of the vortex, a person observing the flow discharging through the opening cannot see that slag is flowing through the opening since molten metal surrounds the slag as it passes through the opening. Rather, in operation of pouring from a ladle to a tundish, for example, the surfacing of slag in the tundish was relied upon to provide an indication that the molten metal had reached the critical level in the ladle, but such an indication occurs only after contamination of the high quality steel in the tundish.
Although sensors have been made to detect the presence of slag in the flow of metal, the continuously changing conditions including turbulence and heat, have affected the ability of previously known sensors to provide highly accurate or reliable responses to the presence of slag in the flow. The sensor signals must also be amplified, filtered and digitally processed by a measuring and control unit. Unavoidable drifts in sensor response are caused by temperature changes and must be compensated. As a result, merely improving the sensitivity of the previously known sensors does not provide an improved result of reducing contamination while limiting the amount of quality metal that must be retained in the vessel.